In combined cycle power systems, the exhaust heat of a gas turbine is recovered in a heat-recovery steam generator (HRSG), followed by steam turbine expansion. In current practice, cooling of gas turbine hot gas path parts is performed by air extracted from the gas turbine compressor and discharged from the cooled part to the hot gas path. The efficiency of this configuration is reduced because the air performing gas turbine hot gas path part cooling, other than that which cools the turbine first stage nozzle, is extracted from the compressor at one or more stages and expanded to the lower turbine gas path pressures without performing useful work. Furthermore, the conventional current practice of open circuit part cooling using compressor extraction air has the undesirable side effect of reducing the quantity of air available for combustion. Thus, in relation to a machine with less compressor air extraction, the current practice requires a higher combustion temperature for a given fuel flow, which in turn yields higher emissions of oxides of nitrogen (NOx).
In prior installations with closed loop cooling systems, the gas turbine cooling duty provides superheated steam for the Rankine steam bottoming cycle. This arrangement reduces the duty of the superheater in the high pressure section of the HRSG, which leaves more energy available to the high pressure evaporator. As a result, the heat that is transferred from the exhaust gas to the steam cycle with a high temperature difference is increased.
U.S. Pat. No. 4,424,668 describes a closed circuit steam cooled gas turbine system which uses steam generated in a low pressure section of a two pressure Rankine steam bottoming cycle. In this system, energy removed from the gas turbine is applied in the low level section of the bottoming cycle, thereby compromising the ability to achieve high thermal efficiency.